Analysis of the selected literature revealed that preferences of viruses and bacteria for low and high humidity are known, while fungi prefer humidity’s above 80%RH for optimal survival on surfaces. For airborne microbes, literature revealed, that midrange humidity was least favourable for survival. Interventional clinical trials by use of humidifiers were analysed for rates for respiratory infections and absenteeism.

While off-gassing of formaldehyde and chemical interactions increases above 40%RH, the concentration of irritating ozone decreases.

Conclusion

The authors conclude that the optimal humidity range for minimizing risks to human health by biological contaminants and chemical interactions, is in the narrow range between 40-60%RH, at normal room temperatures.

The review provides good evidence that the optimal indoor humidity level for human health is 40-60%RH.

To the present day, Sterling’s and Arundel’s publications are the only reviews that are based on interventional clinical studies (seven successful studies) providing evidence for the preventive effect of humidification on respiratory infections and absenteeism.

This paper was the precursor for Arundel’s publication one year later which presented even more in-depth analysis of adverse and health supportive humidity effects.